You'll be able to significantly change the size of system fonts, change the look of certain buttons and switches throughout the OS, and have finer control over the transparency and zooming effects that some people have found irritating in iOS 7.
There's also a number of fixes specifically for iPhone 5S users. Some people have found that updating to iOS 7.0.6 significantly increased battery drain on the 5S; 7.1 fixes that issue.
Others have found that Touch ID's ability to recognize fingerprints on the first try degraded over time under iOS 7.0.x. Noted Apple blogger John Gruber has been using the iOS 7.1 beta for some time, and he has found that, if anything, it works too quickly now:
@buzz@rentzsch If anything, it works too fast for me now. I often unlock when I just want to wake device to use lock screen camera button.
The biggest issue in iOS 7 has been the so-called "white screen of death," which causes many users' devices to crash on a too-frequent basis. It's been rumored since early February that iOS 7.1 will fix that issue as well — from personal experience and feedback from others using the iOS 7.1 beta for the last several weeks, it does some to be far more stable than iOS 7.0.x in regular usage.
For those with access to a vehicle capable of using CarPlay — which won't be many people, considering it's only now being released in new cars — that feature will now be available as well.